316. The view thus afforded of the operations, in their nascent state, which have shaped out and fashioned the present surface of the land, is necessary to prepare us for following them to the utmost extent of their effects. From these effects, the truth of the proposition, that rivers have cut and formed, not the beds only, but the whole of the valleys, or rather system of valleys, through which they flow, is demonstrated on a principle which has a close affinity to that on which chances are usually calculated, § 99. In order to conceive rightly the course of a great river, and the communication subsisting between the main trunk and its remotest branches, let us take the instance of the Danube and cast our eyes on one of the maps constructed by MARSIGLI, for illustrating the natural history of that great river.[163] When it is considered, that over all the vast and uneven surface, which reaches from the Alps to the Euxine, and from the mountains of Crapack to those of Hæmus, a regular communication is kept up between every point and the line of greatest depression, in which the river flows, no one can hesitate to acknowledge, that it is the agency of the waters alone which has opened them a free passage through all the intricacies of this amazing labyrinth. In effect, suppose this communication to be interrupted, and that some sudden operation of nature were to erect a barrier of mountains to oppose the Theise or the Drave, as they rolled their waters to the Danube. From this what could possibly result, but the damming up of those rivers till their waters were deep, or high enough to find a vent, either under the bases or over the tops of the opposing ridge. Thus there would be formed immense lakes and immense cataracts, which, by filling up what was too low, and cutting down what was too high, would in time restore such a uniform declivity of surface as had before prevailed. Just so in the times that are past, whatever may have been the irregularities of the surface at its first emerging from the sea, or whatever irregularities may have been produced in it by subsequent convulsions, the slow action of the streams would not fail in time to create or renew a system of valleys communicating with one another, like that which we at present behold. Water, in all circumstances, would find its way to the lowest point; though, where the surface was quite irregular, it would not do so till after being dammed up in a thousand lakes, or dashed in cataracts over a thousand precipices. Where neither of these is the case; and where the lake and the cataract are comparatively rare phenomena; there we perceive that constitution of a surface, which water alone, of all physical agents, has a tendency to produce; and we must conclude, that the probability of such a constitution having arisen from another cause, is, to the probability of its having arisen from the running of water, in such a proportion as unity bears to a number infinitely great.